The optical switch of the present invention comprises Risley prisms (i.e., pairs of wedge prisms) to redirect light beams from an array of input ports to an array of output ports. The invention further comprises a stepping rotary microactuator for independent rotation of each wedge prism of a Risley prism pair to redirect the light beams.
As the demand for network capacity grows, telecommunications are being increasingly constrained by the need for more bandwidth. Optical fiber is a transmission medium capable of meeting this demand, having the potential in combination with dense wavelength division multiplexing (DWDM) to provide carrying capacity in a single fiber of hundreds of trillions of bits (terabits) per second, far greater than other means suggested for long-distance communications.
However, network transmission speeds and equipment costs are currently severely limited by the requirement of slow and complex electronic switching for signal routingxe2x80x94converting an optical (i.e., photonic) signal from an input optical fiber into an electronic signal, switching the lower speed electronic signal, converting the electronic signal back to an optical signal, and redirecting the optical signal through an output optical fiber. In particular, it is unlikely that such optoelectronic switching will be able to accommodate the large increase in network bandwidth that will accompany the full implementation of DWDM. To fully exploit an optical fiber""s full bandwidth will likely require integrating the transmission, combination, amplification, and switching of optical signals in an all-optical network without optoelectronic switching. Furthermore, efficient switching of terabit optical signals from an input optical fiber array to an array of output optical fibers may require optical cross-connect switches with 256-input x 256-output ports or more.
A number of technologies have been proposed to provide an all-optical switch for telecommunications. These include micromachined tilting mirrors, liquid crystals, bubbles, holograms, and thermo- and acousto-optics. However, none of these technologies are likely to satisfy a wide range of applications, as the requirements for optical fiber array size, scalability, switching speed, reliability, optical loss, cost, and power consumption differ greatly depending on the functionality desired.
In particular, microelectromechanical systems (MEMS) technology has been proposed for optical cross-connects whereby arrays of micromirrors are built on a silicon wafer using surface micromachining fabrication similar to that used in making integrated circuits. These MEMS optical switches use micromirrors to redirect light beams from as many as 256-input to 256-output ports. Each micromirror can be less than 1 millimeter in diameter. However, such a MEMS switch is complex. Furthermore, switching times can be slow and the long-term reliability of moveable parts is a concern. Additionally, the spatial resolution of the MEMS switch may need improvement for some applications. In a large cross-connect switch, the mirrors must be capable of a large range of angular motion, yet be able to accurately move an incident light beam through small tilt angles in order to redirect the incident light beam to a particular output optical fiber and achieve low optical throughput loss. Finally, this MEMS switch requires tightly controlled cleanroom fabrication and contaminant-free switch operation.
Particularly for cross-connect applications, there remains a need for a reliable, scalable, low loss, fast, and low cost optical switch.
According to the present invention, one or more incident light beams from the array of input ports (i.e., input optical fibers) are collimated into an associated array of input Risley prisms (hereinafter termed a Risley prism pair). By independent rotation of the first wedge prism relative to the second wedge prism of each input Risley prism pair, the light beam exiting from any input Risley prism pair can be selectively redirected to any one of an array of output Risley prism pairs. In similar fashion, the wedge prisms of each output Risley prism pair can be independently rotated to direct the light beam into an associated output port (i.e., the output optical fiber) of the optical switch.
Each wedge prism of each input and output Risley prism pair is rotated independently by a rotary microactuator. The rotary microactuator can comprise a cylindrically symmetric electromagnetic stator and an annular soft ferromagnetic rotor that can be patterned with magnetically salient, variable-reluctance pole faces suitable for small angle stepping to provide precise, independent rotation of each wedge prism of a Risley prism pair.
The rotary microactuator can be fabricated by a seven-layer LIGA process (LIGA is a German acronym that stands for lithography, electroplating, and molding), described hereinafter. The seven-layer LIGA process comprises forming sets of stator coil bottoms on a substrate, forming bond pads on the substrate, bonding stator core suspensions to the bond pads, bonding stator coil columns to the stator coil bottoms, bonding stator coil tops to the stator coil columns to form a stator assembly, and bonding a rotor assembly to the substrate within the stator assembly. The rotor assembly is fabricated by forming a torsional spring on a sacrificial substrate and bonding the annular rotor to the torsional spring.
The wedge prisms can be fabricated, integral to the annular rotor of the rotary microactuator, by a deep X-ray lithography (DXRL) process. Using batch-processing techniques, an array of such rotatable wedge prism assemblies can be fabricated on the substrate.